The present invention relates to a supercapacitor comprising a non-aqueous electrolyte and active carbon electrodes of very large specific surface area.
Supercapacitors are energy storage devices having high capacitance per unit mass (of the order of several tens of farads per gram (F/g) of active material to about 100 F/g) and high instantaneous specific power.
A supercapacitor comprises two identical electrodes sandwiching a separator that is permeable to the ions of the electrolyte. There are three different types of supercapacitor depending on the structure of their electrodes and the nature of their electrolyte:
supercapacitors having an organic electrolyte and active carbon electrodes with a large specific surface area lying in the range 1000 m2/g to 3000 m2/g, and which operate electrostatically;
supercapacitors having an aqueous electrolyte and transition metal oxide electrodes, which operate essentially on the basis of surface electrochemical reactions, the mean specific surface area of the oxides used being 100 m2/g; and
supercapacitors having electrodes of electronically conductive polymers such as polypyrrole or polyaniline.
The active carbon electrodes of a conventional non-aqueous electrolyte supercapacitor are thin electrodes obtained by depositing a paste on a current collector. The paste is a mixture of an active material, a diluent, and a binder. Polytetrafluoroethylene (PTFE) is commonly used as the electrode binder.
The binder serves to provide cohesion for the particles of active carbon which is in powder form, but without masking a large fraction of the active surface area.
The binder must also enable the active material to adhere to the current collector.
The binder should confer a certain amount of flexibility to the electrode, particularly while it is being assembled and while it is in operation.
The binder must be inert relative to the components of the electrolyte.
Finally, the insulating binder must enable current to percolate between the grains of active carbon.
In active carbon electrode supercapacitors, it is essential to ensure that the binder does not interact excessively with the grains of active carbon, so as to avoid decreasing the active surface area. A compromise needs to be found so that the binder nevertheless ensures cohesion between the grains of active material.
The object of the present invention is to provide an active carbon electrode supercapacitor having high capacitance per unit mass of the order of about 100 F/g of active material or even more, and having internal resistance that is as low as possible.
Bispo et al. have endeavored (ECS Fall Meeting, October 1995, p. 47) to minimize the internal resistance of a supercapacitor whose electrodes are constituted by a current connector covered in a layer of graphite and then a layer of graphite mixed with Norit(copyright) active carbon.
Norit(copyright) is an active carbon having a mean specific surface area of about 1100 m2/g.
The particles of the active materials, graphite and Norit(copyright) are agglomerated by means of a binder.
Bispo et al. describe that polytetrafluorethylene (PTFE) which is commonly used as a binder gives poor results in terms of internal resistance.
Bispo et al. have shown that cellulose binders such as methylcellulose (MC) or indeed carboxymethylcellulose (CMC) make it possible to reduce the internal resistance of the above-described supercapacitor to a considerable extent.
In particular, a supercapacitor each of whose electrodes contains 15% by weight of CMC presents a capacitance of 60 F/g of active material and an internal resistance of 0.3 xcexa9/dm2.
Seeking to increase the capacitance per unit mass of known supercapacitors having active carbon electrodes, the Applicant has implemented active carbons of high specific surface area, i.e. greater than about 2000 m2/g.
A second object of the Applicant is to provide a 2000 m2/g active carbon electrode supercapacitor that has internal resistance that is as low as possible.
The Applicant has discovered that a binder based on CMC does not give good results with an active carbon of specific surface area greater than about 2000 m2/g because CMC has too great a covering power or film-generating power and therefore masks too large a fraction of the active surface area, such that the internal resistance of the supercapacitor is too high.
The Applicant has observed that a binder based on a styrene/butadiene (SBR) copolymer provides good cohesion to active carbon having a specific surface area greater than about 2000 m2/g, but cannot be implemented because it provides no adhesion to the current collector.
The present invention thus seeks to obtain a supercapacitor having low internal resistance and high capacitance, including a non-aqueous electrolyte and electrode containing:
active carbon of high specific surface area; and
a binder which combines all of the properties discussed above.
That is why the present invention provides a supercapacitor comprising a non-aqueous electrolyte and two carbon electrodes each containing a binder and an electrochemically active material constituted by active carbon having a specific surface area greater than about 2000 m2/g, wherein said binder comprises a mixture of carboxymethylcellulose and a copolymer of styrene and butadiene.
The CMC+SBR binder of the invention makes it possible to improve the capacitance per unit mass of the supercapacitor and to decrease its internal resistance while conserving the adhesive properties of prior art binders such as CMC and PTFE.
The binder of the supercapacitor of the invention advantageously contains a combination of CMC to provide adhesion between the grains of active carbon and a styrene and butadiene copolymer to confer flexibility to the electrode.
The CMC used in the context of the present invention preferably has a mean molecular weight lying in the range about 300,000 and 450,000. Its degree of polymerization is advantageously of the order of 1500 to 2000.
The binder preferably contains 25% to 75% by weight of styrene butadiene copolymer, and 25% to 75% by weight of carboxymethylcellulose relative to 100% by weight of the two polymers.
The electrode advantageously contains at least 80% by weight active carbon and 2% to 20% by weight of binder, and preferably 4% to 10% by weight of binder.
Said electrode can be obtained by the method described below.
The first step of the method consists in fabricating a paste from active carbon and binder put into solution in water.
The following steps consist in spreading the paste on a metal foil made of aluminum, followed by drying in an oven, and then calendaring to obtain the desired porosity, greater than 50% and preferably about 70% to 80%.
The non-aqueous electrolyte of the supercapacitor of the invention contains a solution of a conductive salt dissolved in a solvent selected from: propylene carbonate, ethylene carbonate, butylene carbonate, gamma-butyrolactone, gamma-valerolactone, acetonitrile, propionitrile, glutaronitrile, adiponitrile, methoxyacetonitrile, N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, N-methyloxazolidinoe, nitromethane, nitroethane, sulfonane, 3-methylsulfonane, dimethyl-sulfoxide, trimethylphosphate, and mixtures thereof.
The conductive salt is preferably a quaternary ammonium salt, the anion being selected from BFxe2x88x924 and PFxe2x88x926. The quaternary ammonium is preferably selected from Me4N+, Et4N+, Pr4N+, and Bu4N+.